In a Martian oxymoron, icy clouds could have kept the Red Planet warm enough for life to thrive

Hard to believe, but the reddish expanses of frozen desert on Mars were not always so barren. Perseverance is crawling through a crater that is thought to have once been an ancient river delta (above) in which unknown things might have been swimming.

How Mars managed to stay warm enough to be habitable for hundreds of years has been a mystery, until now, perhaps. Before it became a sun-blasted wasteland, it only received a third of the sunlight that shines down on Earth, which would have made it (at least by Earthling standards) hardly warm enough for any hypothetical life to thrive. University of Chicago planetary scientist Edwin Kite may have the answer. He believes it was all in the clouds.

"To test the cloud greenhouse hypothesis, we used the MarsWRF Global Climate Model, modified to include radiatively active water ice clouds," Kite who led a study recently published in PNAS, told SYFY WIRE. "We simulated a dynamic water cycle, including sedimentation of individual cloud particles, rapid snow-out above an autoconversion threshold, and exchange with surface water ice."

High in the upper atmosphere of Mars, icy clouds could have caused a greenhouse effect to trap just enough warmth for anything that might have once lived there to potentially survive. Though scientists have known about liquid water in the Red Planet’s past, the lack of sun had them questioning how it somehow was able to keep flowing instead of freezing. Clouds in the upper atmosphere, like cirrus clouds on Earth, offered a possible explanation. The only way to really see if high-altitude clouds could have acted as a sort of sweatshirt for Mars was to test the hypothesis virtually.

NASA animation of clouds on Mars throughout the day. Credit: NASA

Turned out that the reason temperatures on Mars were not so extreme for a while was because of the effect of ground ice on the clouds above. Too much ice would mean thicker clouds born of all the surface humidity caused by evaporating water from lakes, rivers, and melting ice. Clouds like these don’t really help insulate a planet. Never mind that hey reflect the sunlight needed to keep temperatures bearable. High-altitude clouds that would make effective insulators would only form if the planet was much drier, because less evaporation means less humidity.

"Through trial and error, we noticed that simulations with extensive water ice led to little or no warming, whereas simulations with patchy surface water ice led to strong, stable warming," Kite said.

The global climate model (GCM) that Kite and his team used to prove this revealed that the greenhouse effect from icy clouds in the upper atmosphere was possible because Mars was covered in patches of liquid and frozen water, as opposed to the water that is all over Earth. Ancient water on Mars moved slowly compared to our planet’s superfast cycles that flood some places and leave others parched. Martian water vapor was not in such a rush. When it floated up to the atmosphere, it would hang around and form protective clouds.

So what could that give away about Jezero Crater, which Pereseverance would probably be submerged in if it had landed there when Mars was much warmer?

"Finding impact craters interbedded within the delta would show that the lake lasted a long time, which would help to constrain climate models," Kite said. "If the impact craters are small, that would imply that the atmosphere was thin, again putting limits on climate models."

Much of the ice on Mars sublimates, or vaporizes without melting, when temperatures rise. What Kite suggested is that the resulting vapor from ice particles that grew heavy enough to fall would be swept back up by winds to maintain the clouds. This phenomenon would mean that the climate of Mars was once warm and mostly dry, with groundwater or ice melt seeping into lakes and rivers. Something like this could be useful in the search for life. Mars is the only planet we know of that once had conditions that could have been ideal for breeding life, but then lost everything when its atmosphere was annihilated.

Kite believes that what we find out about what remains of Mars, which now appears to be a pile of red rocks pockmarked with craters, might tell us more about the rock we live on. He also thinks it could also give us a preview of the potential for habitability on other planet. In new field of comparative planteray habitability, scientists will see ancient Mars as one basis for comparison to exoplanets in the quest for extraterrestrial life that humans have pursued since we knew there was a vast expanse beyond Earth. 

"Earth’s long-term climate stability is remarkable," he said. "We want to understand all the ways in which a planet’s longterm climate stability can break down and all of the ways (not just Earth’s way) that it can be maintained. Mars’
record of planetary habitability greatly ameliorates the survivorship
bias that inherently limits using Earth as an analog to understand the
habitability of exoplanets."

Even if Earth is a cosmic anomaly, maybe a planet that could pass for what Mars was in its earlier days does exist out there.